Method and apparatus for maintaining cell wall integrity during thermal runaway using an outer layer of intumescent material

ABSTRACT

A method and apparatus is provided in which a layer of an intumescent material surrounds the casing of a battery, the layer helping to prevent the formation of perforations in the battery casing during a thermal runaway event and, if a perforation is formed, inhibiting the flow of hot, pressurized gas from within the battery. A sleeve, surrounding the cell, may be used to contain the intumescent material during the thermal event.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/504,712, filed Jul. 17, 2009, the disclosure of which is incorporatedherein by reference for any and all purposes.

FIELD OF THE INVENTION

The present invention relates generally to batteries, and moreparticularly, to a means for maintaining cell wall integrity duringthermal runaway.

BACKGROUND OF THE INVENTION

Batteries can be broadly classified into primary and secondarybatteries. Primary batteries, also referred to as disposable batteries,are intended to be used until depleted, after which they are simplyreplaced with one or more new batteries. Secondary batteries, morecommonly referred to as rechargeable batteries, are capable of beingrepeatedly recharged and reused, therefore offering economic,environmental and ease-of-use benefits compared to a disposable battery.

Although rechargeable batteries offer a number of advantages overdisposable batteries, this type of battery is not without its drawbacks.In general, most of the disadvantages associated with rechargeablebatteries are due to the battery chemistries employed, as thesechemistries tend to be less stable than those used in primary cells. Dueto these relatively unstable chemistries, secondary cells often requirespecial handling during fabrication. Additionally, secondary cells suchas lithium-ion cells tend to be more prone to thermal runaway thanprimary cells, thermal runaway occurring when the internal reaction rateincreases to the point that more heat is being generated than can bewithdrawn, leading to a further increase in both reaction rate and heatgeneration. Eventually the amount of generated heat is great enough tolead to the combustion of the battery as well as materials in proximityto the battery. Thermal runaway may be initiated by a short circuitwithin the cell, improper cell use, physical abuse, manufacturingdefects, or exposure of the cell to extreme external temperatures.

Thermal runaway is of major concern since a single incident can lead tosignificant property damage and, in some circumstances, bodily harm orloss of life. When a battery undergoes thermal runaway, it typicallyemits a large quantity of smoke, jets of flaming liquid electrolyte, andsufficient heat to lead to the combustion and destruction of materialsin close proximity to the cell. If the cell undergoing thermal runawayis surrounded by one or more additional cells as is typical in a batterypack, then a single thermal runaway event can quickly lead to thethermal runaway of multiple cells which, in turn, can lead to much moreextensive collateral damage. Regardless of whether a single cell ormultiple cells are undergoing this phenomenon, if the initial fire isnot extinguished immediately, subsequent fires may be caused thatdramatically expand the degree of property damage. For example, thethermal runaway of a battery within an unattended laptop will likelyresult in not only the destruction of the laptop, but also at leastpartial destruction of its surroundings, e.g., home, office, car,laboratory, etc. If the laptop is on-board an aircraft, for examplewithin the cargo hold or a luggage compartment, the ensuing smoke andfire may lead to an emergency landing or, under more dire conditions, acrash landing. Similarly, the thermal runaway of one or more batterieswithin the battery pack of a hybrid or electric vehicle may destroy notonly the car, but may lead to a car wreck if the car is being driven orthe destruction of its surroundings if the car is parked.

One approach to overcoming this problem is by reducing the risk ofthermal runaway. For example, to prevent batteries from being shortedout during storage and/or handling, precautions can be taken to ensurethat batteries are properly stored, for example by insulating thebattery terminals and using specifically designed battery storagecontainers. Another approach to overcoming the thermal runaway problemis to develop new cell chemistries and/or modify existing cellchemistries. For example, research is currently underway to developcomposite cathodes that are more tolerant of high charging potentials.Research is also underway to develop electrolyte additives that formmore stable passivation layers on the electrodes. Although this researchmay lead to improved cell chemistries and cell designs, currently thisresearch is only expected to reduce, not eliminate, the possibility ofthermal runaway. Accordingly, what is needed is a means for maintainingcell integrity during a thermal runaway event, thereby minimizing damageto adjacent cells and materials. The present invention provides such ameans.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for inhibiting theformation of perforations in the battery casing during a thermal runawayevent.

In at least one embodiment of the invention, a battery assembly isprovided comprised of a battery and a layer of an intumescent materialsurrounding an outer surface of the battery cell case. The intumescentmaterial may be comprised of graphite-based intumescent materials,thermoplastic elastomers, ceramic-based intumescent materials,vermiculite/mineral fiber based intumescent materials, and ammoniumpolyphosphate based intumescent materials. Preferably the intumescentmaterial comprising the layer has a start expansion temperature in therange of 100° C. to 300° C., and more preferably in the range of 200° C.to 300° C. Preferably the intumescent material is biologically inert.The battery assembly may further comprise a sleeve that surrounds theintumescent layer and is spaced apart from the intumescent layer.Preferably the sleeve is comprised of a material with a yield strengthof at least 75 MPa; and/or a yield strength of at least 150 MPa; and/ora yield strength of at least 250 MPa. The battery assembly may furthercomprise at least one spacer ring that separates the interior surface ofthe sleeve with the exterior surface of the intumescent material. Thebattery assembly may further comprise a second layer of the intumescentmaterial, which may be integral with the layer of intumescent materialsurrounding the outer surface of the cell case, wherein the second layercovers the cell case bottom.

In at least one embodiment of the invention, a method of preventing theformation of a perforation in the outer surface of a battery case duringa thermal runaway event is provided, the method comprising the step ofcovering the outer surface of the battery case in a layer of anintumescent material, wherein the covering step is performed prior tothe thermal runaway event. The method may further comprise the step ofsurrounding the layer of intumescent material with a sleeve, wherein thesleeve is separated from the intumescent layer by a preset distance. Thecovering step may comprise the step of coating the outer surface of thecell case with the intumescent material. The covering step may furthercomprise the step of forming a strip of the intumescent material andwrapping the strip around the outer surface of the battery case, whichmay be bonded in place.

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross-sectional illustration of a cell utilizingthe 18650 form-factor;

FIG. 2 illustrates the cell shown in FIG. 1, modified to increasefailure resistance in accordance with the prior art approach;

FIG. 3 illustrates a preferred embodiment of the invention utilizing ahigh strength sleeve;

FIG. 4 illustrates an alternate embodiment utilizing multiple highstrength sleeves;

FIG. 5 illustrates an alternate embodiment utilizing an inner, thermallyinsulating layer and an outer, high strength layer;

FIG. 6 illustrates a modification of the configuration shown in FIG. 5,the modified configuration including multiple layers that alternatebetween thermally insulating material and high strength material;

FIG. 7 illustrates an alternate embodiment utilizing a layer ofthermally insulating material interposed between the electrode assemblyand the inner surface of the cell casing;

FIG. 8 illustrates an alternate embodiment utilizing a high heatcapacity layer;

FIG. 9 illustrates an alternate embodiment utilizing a coating of anintumescent material applied to the outer surface of the battery casing;

FIG. 10 illustrates a modification of the configuration shown in FIG. 9,in which an outer containment sleeve is positioned around saidintumescent coated cell;

FIG. 11 illustrates a modification of the configuration shown in FIG. 3,the modified configuration including a multi-piece design that coversthe end surface of the cell; and

FIG. 12 illustrates a modification of the configuration shown in FIG. 3,the modified configuration including a single piece design that coversthe end surface of the cell.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

In the following text, the terms “battery”, “cell”, and “battery cell”may be used interchangeably and may refer to any of a variety ofdifferent cell chemistries and configurations including, but not limitedto, lithium ion (e.g., lithium iron phosphate, lithium cobalt oxide,other lithium metal oxides, etc.), lithium ion polymer, nickel metalhydride, nickel cadmium, nickel hydrogen, nickel zinc, silver zinc, orother battery type/configuration. The term “battery pack” as used hereinrefers to multiple individual batteries contained within a single pieceor multi-piece housing, the individual batteries electricallyinterconnected to achieve the desired voltage and capacity for aparticular application. It should be understood that identical elementsymbols used on multiple figures refer to the same component, orcomponents of equal functionality. Additionally, the accompanyingfigures are only meant to illustrate, not limit, the scope of theinvention and should not be considered to be to scale.

FIG. 1 is a simplified cross-sectional view of a conventional battery100, for example a lithium ion battery utilizing the 18650 form-factor.Battery 100 includes a cylindrical case 101, an electrode assembly 103,and a cap assembly 105. Case 101 is typically made of a metal, such asnickel-plated steel, that has been selected such that it will not reactwith the battery materials, e.g., the electrolyte, electrode assembly,etc. Typically cell casing 101 is fabricated in such a way that thebottom surface 102 is integrated into the case, resulting in a seamlesslower cell casing. The open end of cell case 101 is sealed by a capassembly 105, assembly 105 including a battery terminal 107, e.g., thepositive terminal, and an insulator 109, insulator 109 preventingterminal 107 from making electrical contact with case 101. Although notshown, a typical cap assembly will also include an internal positivetemperature coefficient (PTC) current limiting device, a currentinterrupt device (CID), and a venting mechanism, the venting mechanismdesigned to rupture at high pressures and provide a pathway for cellcontents to escape. Additionally, cap assembly 105 may contain otherseals and elements depending upon the selected design/configuration.

Electrode assembly 103 is comprised of an anode sheet, a cathode sheetand an interposed separator, wound together in a spiral pattern oftenreferred to as a ‘jellyroll’. An anode electrode tab 111 connects theanode electrode of the wound electrode assembly to the negative terminalwhile a cathode tab 113 connects the cathode electrode of the woundelectrode assembly to the positive terminal. In the illustratedembodiment, the negative terminal is case 101 and the positive terminalis terminal 107. In most configurations, battery 100 also includes apair of insulators 115/117. Case 101 includes a crimped portion 119 thatis designed to help hold the internal elements, e.g., seals, electrodeassembly, etc., in place.

In a conventional cell, such as the cell shown in FIG. 1, a variety ofdifferent abusive operating/charging conditions and/or manufacturingdefects may cause the cell to enter into thermal runaway, where theamount of internally generated heat is greater than that which can beeffectively withdrawn. As a result, a large amount of thermal energy israpidly released, heating the entire cell up to a temperature of 900° C.or more and causing the formation of localized hot spots where thetemperature may exceed 1500° C. Accompanying this energy release is therelease of gas, causing the gas pressure within the cell to increase.

To combat the effects of thermal runaway, a conventional cell willtypically include a venting element within the cap assembly. The purposeof the venting element is to release, in a somewhat controlled fashion,the gas generated during the thermal runaway event, thereby preventingthe internal gas pressure of the cell from exceeding its predeterminedoperating range.

While the venting element of a cell may prevent excessive internalpressure, this element may have little effect on the thermal aspects ofa thermal runaway event. For example, if a local hot spot occurs in cell100 at a location 121, the thermal energy released at this spot may besufficient to heat the adjacent area 123 of the single layer casing wall101 to above its melting point. Even if the temperature of area 123 isnot increased beyond its melting point, the temperature of area 123 inconcert with the increased internal cell pressure may quickly lead tothe casing wall being perforated at this location. Once perforated, theelevated internal cell pressure will cause additional hot gas to bedirected to this location, further compromising the cell at this andadjoining locations.

It should be noted that when a cell undergoes thermal runaway and ventsin a controlled fashion using the intended venting element, the cellwall may still perforate due to the size of the vent, the materialcharacteristics of the cell wall, and the flow of hot gas travelingalong the cell wall as it rushes towards the ruptured vent. Once thecell wall is compromised, i.e., perforated, collateral damage canquickly escalate, due both to the unpredictable location of such a hotspot and due to the unpredictable manner in which such cell wallperforations grow and affect neighboring cells. For example, if the cellis one of a large array of cells comprising a battery pack, the jet ofhot gas escaping the cell perforation may heat the adjacent cell toabove its critical temperature, causing the adjacent cell to enter intothermal runaway. Accordingly, it will be appreciated that theperforation of the wall of one cell during thermal runaway can initiatea cascading reaction that can spread throughout the battery pack.Furthermore, even if the jet of hot gas escaping the cell perforationfrom the first cell does not initiate thermal runaway in the adjacentcell, it may still affect the health of the adjacent cell, for exampleby weakening the adjacent cell wall, thereby making the adjacent cellmore susceptible to future failure.

As previously noted, cell perforations are due to localized, transienthot spots where hot, pressurized gas from a concentrated thermal eventis flowing near the inner surface of the cell. Whether or not a celltransient hot spot perforates the cell wall or simply dissipates andleaves the cell casing intact depends on a number of factors. Thesefactors can be divided into two groups; those that are based on thecharacteristics of the thermal event and those that are based on thephysical qualities of the cell casing. Factors within the first groupinclude the size and temperature of the hot spot as well as the durationof the thermal event and the amount of gas generated by the event.Factors within the second group include the wall thickness as well asthe casing's yield strength as a function of temperature, heat capacityand thermal conductivity.

FIG. 2 illustrates the conventional approach to improving the failureresistance of a cell, where failure is defined as a thermally inducedwall perforation. As shown, in cell 200 the thickness of casing 201 hasbeen significantly increased, thereby improving the cell's failureresistance at the expense of cell weight. Assuming that cell mass is notan issue, which it is not for many consumer applications where only afew cells are used, the conventional approach to preventing wallperforations during thermal runaway is quite effective. Unfortunately,for those applications in which the battery pack may include hundreds oreven thousands of cells, for example the battery pack of an electricvehicle, the added mass of this approach is very unattractive sinceperformance is directly tied to mass. For instance, if the conventionalapproach only adds 4 grams per cell, for a battery pack with 10,000cells, this increase adds up to 40 kg. Accordingly, for theseapplications the conventional approach to improving cell failureresistance is unacceptable.

In addition to recognizing the weight constraints placed on thebatteries within a large battery pack and the factors that contribute tothe initiation and growth of wall perforations during thermal runaway,the present inventors also recognize that once a cell enters intothermal runaway, it is no longer viable. Accordingly, at this point theprimary purpose of the cell casing is to control the direction andpathway for the hot, escaping gas generated by the thermal runawayevent. In recognition of these design parameters, the intent of thepresent invention is to minimize, if not altogether eliminate, theescape of hot, pressurized gas from the sides of a cell where theescaping gas can adversely affect neighboring cells. Rather than allowthe hot gas to escape through wall perforations, the present inventionforces this gas to exit the cell from either end surface, or in someembodiments, from only one of the cell end surfaces.

FIG. 3 illustrates one embodiment of the invention. In this embodiment,a conventional cell such as that shown in FIG. 1 is modified by slidinga sleeve 301 lengthwise over the outer casing wall 101. Typically sleeve301 is added after fabrication of the cell has been completed, but priorto the inclusion of the cell within the cell's intended application,e.g., a battery pack. Sleeve 301 may be added by the cell manufactureror added by another party. Preferably there is minimal clearance betweenthe inner surface of sleeve 301 and the outer surface of cell casing101. In at least one embodiment, sleeve 301 is press-fit, also commonlyreferred to as a force fit, onto case 101, for example using a hydraulicpress-fit system.

The use of a separate sleeve offers a number of advantages over theconventional approach of simply increasing the wall thickness of casing101. First and foremost, the goals of significantly decreasing the riskof hot, pressurized gas escaping through the cell wall and redirectingthis escaping gas to the cell ends are both achieved while adding muchless weight to the cell than would be required to achieve the sameperformance using the conventional, wall-thickening approach. Second, assleeve 301 is preferably added after completion of cell processing, thisapproach can be used with virtually any manufacturer's cell since itdoes not affect cell manufacturing. Third, while the material selectedfor casing 101 must be non-reactive with the cell contents (e.g.,electrolyte and electrode assembly), no such material constraints areplaced on sleeve 301. Accordingly the material used for sleeve 301 canbe selected based on its ability to minimize or eliminate the escape ofhot, pressurized gas from the cell sidewalls.

During thermal runaway, when a transient hot spot (e.g., spot 121 inFIG. 1) develops, sleeve 301 performs several functions. Initially, itdraws off heat from the cell case 101 at the location adjacent to thetransient hot spot (e.g., location 123 of case 101 in FIG. 1), therebyslowing down the perforation of the wall. If a perforation in casing 101starts, then sleeve 301 prevents the immediate and rapid growth of theperforation that would otherwise occur due to the pressurized hot gasbeing directed through the perforation. Additionally, due to the thermalcontact resistance between case 101 and sleeve 301, and given thelimited duration of the thermal event, sleeve 301 is more resistive toperforation than case 101. The failure resistance of sleeve 301 isfurther enhanced by fabricating the sleeve from a material that exhibitshigh yield strength at high temperature. Preferably the yield strengthof sleeve 301 is at least 250 MPa at room temperature, more preferablyat least 250 MPa at a temperature of 1000° C., still more preferably atleast 500 MPa at room temperature, and yet still more preferably atleast 500 MPa at a temperature of 1000° C.

Given the thermal contact resistance between the case and the sleeve,and given that the sleeve is preferably constructed of a material withthe desired material characteristics, typically only casing 101 will beperforated during thermal runaway, leaving outer sleeve 301 intact. As aresult, adjacent cells are not subjected to a stream of hightemperature, pressurized gas. Additionally, in this situation, and giventhe minimal clearance between the outer surface of case 101 and theinner surface of sleeve 301, sleeve 301 prevents the rapid growth of thecase wall perforation, thereby minimizing the amount of gas that escapesthrough the case wall perforation. This effect is aided by the cell casedimensions expanding slightly during thermal runaway due to theincreased internal pressure, thereby further improving the contactbetween the outer cell surface and the interior surface of the sleeve.Accordingly little, if any, hot gas escapes through the case wallperforation and that which does escape is redirected by sleeve 301 toeither cell end where its effects on neighboring cells are minimized.

It will be appreciated that there are a variety of materials suitablefor use in constructing sleeve 301. Exemplary materials include variousengineering and high strength structural steels, plated steel, stainlesssteel, titanium and titanium alloys, and nickel alloys. The preferredtechnique or techniques for fabricating sleeve 301 depend on theselected material as well as the shape of the cell for which the sleeveis to be used. For example, if the cell has the 18650 form-factor,sleeve 301 is cylindrically shaped. Sleeve 301 may be fabricated usingany of a variety of techniques including, but not limited to, (i)drawing the sleeve, (ii) wrapping a strip or sheet of the desiredmaterial around the cell and welding or mechanically coupling the edgestogether, (iii) bending and welding a strip or sheet of the desiredmaterial into the desired shape, or (iv) bending a strip or sheet of thedesired material into the desired shape and coupling the two edgestogether using mechanical interconnects. Sleeve 301 may be bonded intoplace.

FIG. 4 illustrates an alternate embodiment of the invention. In thisembodiment, a conventional cell such as that shown in FIG. 1 is modifiedby wrapping the cell with a plurality of layers 401-403. Preferablythree layers are used, as shown, although it will be appreciated thateither a fewer number or a greater number of layers can be used in thisembodiment. Preferably all of the layers are comprised of a materialexhibiting high yield strength at high temperatures, more specifically ayield strength of at least 250 MPa at room temperature, still morepreferably at least 250 MPa at a temperature of 1000° C., still morepreferably at least 500 MPa at room temperature, and yet still morepreferably at least 500 MPa at a temperature of 1000° C. The pluralityof layers may be comprised of the same material, or of one or moredifferent materials. Exemplary materials include various engineering andhigh strength structural steels, plated steel, stainless steel, titaniumand titanium alloys, and nickel alloys.

The inventors have found that due to the thermal contact resistancebetween layers 401-403, in addition to the thermal contact resistancebetween casing 101 and the innermost layer 401, the use of multiplelayers is more effective than a single layer sleeve as shown in FIG. 3.Following this approach, and even using the same material, the additivethickness of layers 401-403 can be less than the thickness of sleeve 301while achieving the same level of perforation resistance. For example,in one test the inventors found that three 25 micron thick layers ofstainless steel were just as effective as a single, 100 micron thicklayer of stainless steel. Accordingly, further mass reductions can beachieved by encasing the cell in multiple layers as opposed to a singlelayer sleeve.

As in the previous embodiment, there is minimal clearance between thelayers as well as between the innermost layer and the outer surface ofcell casing 101. The layers of this embodiment can be fabricated asindividual sleeves of gradually increasing inside diameter or, aspreferred, fabricated from a single strip or sheet that is wrappedmultiple times around the outside of the cell case. The layers, and/orthe final, outermost edge of the outermost layer, can be bonded, welded,or held in place with mechanical interconnects.

As in the prior embodiment, the layers of the present embodiment controlthe growth of any cell wall perforations while preventing hot,pressurized gas from being directed at adjacent cells.

In an alternate preferred embodiment of the invention, a pair of layers(FIG. 5), or multiple pairs of layers (FIG. 6), surround the batterycase (e.g., cell case 101 in FIG. 1). Although the embodiment shown inFIG. 6 only includes two sets of layers, it will be appreciated thatadditional layer pairs can be used with the invention.

The innermost layer of each layer pair, e.g., layer 501 in FIG. 5 andeach layer 601 in FIG. 6, is comprised of a thermal insulator,preferably a light weight thermal insulator. The outermost layer of eachlayer pair, e.g., layer 503 in FIG. 5 and each layer 603 in FIG. 6, iscomprised of a high yield strength material. Preferably the high yieldstrength material used in layer 503/603 has a yield strength of at least250 MPa at room temperature, more preferably at least 250 MPa at atemperature of 1000° C., still more preferably at least 500 MPa at roomtemperature, and yet still more preferably at least 500 MPa at atemperature of 1000° C. Exemplary high yield strength materials include,but are not limited to, various engineering and high strength structuralsteels, plated steel, stainless steel, titanium and titanium alloys, andnickel alloys.

In one configuration, the material used for the thermal insulatorlayers, e.g., layer 501 in FIG. 5 and each layer 601 in FIG. 6, iscapable of withstanding a relatively high temperature, for example,capable of withstanding temperatures of more than 500° C. continuouslyand/or withstanding temperatures of more than 1000° C. for a period ofat least 10 seconds and/or withstanding temperatures of more than 1400°C. for a period of at least 1 second. It will be appreciated that thereare numerous materials and composite materials that can be used tofabricate this layer. Exemplary materials include, but are not limitedto, fiberglass, mineral wool, silica/silica fibers, alumina, Kevlar®,Nomex®, calcium-silicate or calcium-magnesium-silicate fibers, someceramics, etc.

In an alternate configuration, the material used for the thermalinsulator layers, e.g., layer 501 in FIG. 5 and each layer 601 in FIG.6, is an insulator with a relatively low melting temperature, forexample, with a melting temperature of less than a 500° C., morepreferably less than 350° C., and still more preferably less than 250°C. Exemplary materials include, but are not limited to, plastics, morespecifically a polymer such as polyethylene or polypropylene.

During a thermal runaway event, when a transient hot spot is formed(e.g., spot 121 of FIG. 1), the thermally insulating layer (e.g., layer501) significantly reduces the temperature of the high yield strengthlayer (e.g., layer 503) adjacent to the hot spot, thereby drasticallyreducing the risk of a perforation forming in the outermost, high yieldstrength layer. The use of two or more layer pairs as shown in FIG. 6further reduces this risk by requiring that the hot, pressurized gasfrom the transient hot spot ablate two or more thermally insulatinglayers and two or more high yield strength layers. It will beappreciated that by lowering the temperature near the transient hotspot, the high yield strength layer can be thinner than would otherwisebe required, resulting in further weight gains. In this embodiment, oneof the primary purposes of the high yield strength layer (e.g., layer501, layer 601) is to provide a strong support layer for the thermalinsulator, thereby preventing its rapid ablation by the pressurized, hotgas.

The thermally insulating layer (e.g., layer 501, layers 601) can befabricated in a variety of ways, depending upon the material ormaterials used to fabricate the layer. For example, this layer can beformed by bonding or otherwise attaching a thin layer of suitablethermal insulator to the cell case, or to the underlying high yieldstrength layer in the configuration utilizing multiple layer pairs. Thisthermally insulating layer may be preformed using a weaving, machining,or other technique. Alternately, this layer can be fabricated byrolling, dipping or spraying the cell, or the underlying high yieldstrength layer, with the thermally insulating material. Alternately,this layer can be deposited on the inner surface of the high strengthlayer (e.g., layer 503, layers 603) prior to the assembly of the highstrength layer onto the cell. As previously described relative to layers301 and 401-403, high strength layer 503 can be fabricated as a sleeveor as a strip or sheet of suitable material that is bent to shape andheld together by welding, bonding or mechanical interconnects.

In a modification of the embodiment shown in FIG. 5, a layer ofthermally insulating material is interposed between the inside wallsurface of case 100 and the exterior surface of the electrode assembly(FIG. 7). As in the prior embodiments shown in FIGS. 5 and 6, thethermally insulating layer 701 shown in FIG. 7 minimizes the transfer ofthermal energy from the transient hot spot to the next layer which, inthis configuration, is outer cell casing 101. Given the relatively shortduration of the thermal event, the delay provided by layer 701 issufficient to prevent the pressurized hot gas accompanying the transienthot spot from perforating casing 101. In this embodiment, case 101provides the strength and support necessary to ensure that the region oflayer 701 adjacent to the hot spot is not rapidly ablated away, therebyeliminating its usefulness.

Although the use of a single, light weight, thermally insulating layer701 as shown in FIG. 7 can dramatically improve the cell's resistance towall perforations during thermal runaway, the inclusion of such a layeris not without its drawbacks. Primarily, the material or materialsselected for use in layer 701 must be non-reactive with the electrolyteand electrode assembly. Additionally, the inclusion of layer 701requires modifying the cell fabrication and assembly process.

As in the previous embodiment, preferably thermally insulating layer 701is fabricated from a thermal insulator that is capable of withstandingtemperatures of more than 500° C. continuously and/or withstandingtemperatures of more than 1000° C. for a period of at least 10 secondsand/or withstanding temperatures of more than 1400° C. for a period ofat least 1 second. Exemplary materials include, but are not limited to,fiberglass, mineral wool, silica/silica fibers, alumina, Kevlar®,Nomex®, calcium-silicate or calcium-magnesium-silicate fibers, someceramics, etc.

Preferably layer 701 is formed by depositing or otherwise coating theinside surface of case 101 with the selected thermal insulator, thisdeposition/coating step being performed prior to assembling theelectrode assembly within the cell casing. Alternately, the exteriorsurface of the electrode assembly may be deposited or otherwise coatedwith the selected thermal insulator, this deposition/coating step beingperformed prior to assembling the electrode assembly within the cellcasing. Alternately, a sheet/strip of the selected thermal insulator canbe formed, after which the sheet/strip is wrapped around the outsidesurface of the electrode assembly before assembly within the cellcasing. Alternately, a sleeve of the selected thermal insulator can beformed, with or without a bottom surface, and inserted within the cellcase prior to final assembly.

In a variation of the embodiment described above, layer 701 is notformed from a thermal insulator, but formed from the same separator, ora similar separator, as that used during the fabrication of theelectrode assembly (i.e., assembly 103). By using the same separator,fabrication is simplified as the winding process that is used to createthe electrode jellyroll can be used to create the additional separatorlayers, simply by lengthening the separator material relative to thelengths of the anode and cathode materials. Typically the separator isfabricated from a thermoplastic, such as polyethylene, polypropylene orsome combination of the two. In a slight modification of thisembodiment, in addition to wrapping extra layers of separator around thejellyroll, layers of one or more metals are also wrapped around thejellyroll. The metal or metals within the extra layers may be the samemetals used to form the cathode and/or anode of the electrode assembly,as long as these additional metal layers are not part of the activeelectrode assembly.

During a thermal runaway event, the additional layers of separator, orthe additional layers of the separator plus metal layer(s), absorb someof the energy released during the thermal runaway event, therebyinhibiting the formation of perforations in the cell case wall. Theseextra separator layers, with or without the additional non-active metallayer(s), also help to prevent short circuits from occurring between theelectrode assembly and the cell case.

In another variation of the embodiment described above, layer 701 is notformed from a thermal insulator or from a separator, but formed from atleast one layer of metal. The at least one layer of metal isnon-reactive with the electrode assembly, including the electrolyte, andis not an active element of the electrode assembly. An exemplary metalfor at least some cell chemistries is steel, or a steel alloy.Preferably this layer or layers of metal (e.g., layer 701) is woundaround the electrode assembly prior to inserting the assembly into cellcasing 101.

FIG. 8 illustrates an alternate preferred embodiment of the inventionutilizing the addition of a layer 801 surrounding the outer surface ofthe cell case, layer 801 comprised of a material with a latent heat offusion of at least 200 kJ/kg, more preferably with a latent heat offusion of at least 300 kJ/kg, and still more preferably with a latentheat of fusion of at least 350 kJ/kg. Preferably the material has amelting point between 300° C. and 1500° C., and more preferably between500° C. and 1200° C. Although a variety of materials can be used forlayer 801, preferably layer 801 is fabricated from aluminum or analuminum alloy.

During thermal runaway, when a transient hot spot (e.g., spot 121 inFIG. 1) develops, layer 801 rapidly draws off the heat from the cellcase 101 at the location adjacent to the transient hot spot. By rapidlydrawing off the heat, the temperature in this region of the case (e.g.,location 123 of case 101 in FIG. 1) never reaches its melting point orbecomes so weak that the internal cell pressure can perforate the cellcasing at this location. It will be appreciated that although a portionof layer 801 adjacent to the transient hot spot may melt during thisprocess, given the relatively short duration of the event, and giventhat layer 801 is not repeatedly subjected to thermal runawayconditions, the loss of this portion of layer 801 via melting does notcompromise the performance of this embodiment.

As with the embodiment illustrated in FIG. 3, layer 801 may be drawn(e.g., as a sleeve), fabricated by bending and welding a strip or sheetinto the desired shape, or fabricated by bending a strip or sheet intothe desired shape and coupling the two edges together using mechanicalinterconnects. Layer 801 may be bonded into place, welded into place,press-fit into place, or otherwise fit to cell case 101. As in theembodiments discussed above and illustrated in FIGS. 3-6, there isminimal clearance between layer 801 and the outer surface of cell casing101.

FIG. 9 illustrates an alternate preferred embodiment of the invention inwhich the outer surface of the cell case is coated with a layer 901 ofan intumescent material. During thermal runaway, the exterior surface ofthe battery heats up, this heating process typically initiating at oneor more transient hot spots (e.g., spot 121 in FIG. 1). Intumescentlayer 901 begins to expand as soon as the material's start expansiontemperature (SET) is reached. Accordingly, layer 901 typically willbegin to expand at a location near the transient hot spots, and thenwill continue to expand as the entire battery heats up past the SETtemperature. After expansion, the intumescent material of layer 901hardens. Intumescent layer 901 helps to prevent the formation of aperforation in the battery case near the transient hot spots and, if oneis formed, helps to contain the escaping hot, pressurized gas.

FIG. 10 illustrates a slight modification of the embodiment shown inFIG. 9. In the modified configuration, an outer sleeve 1001 surroundsthe intumescent covered cell. Preferably sleeve 1001 has a yieldstrength of at least 75 MPa, more preferably at least 150 MPa, and yetstill more preferably at least 250 MPa. The interior surface of sleeve1001 is spaced apart from the intumescent covered cell by a spacing1003, thus insuring that intumescent layer 901 has room to expand. Atthe same time, sleeve 1001 helps to prevent layer 901, after expansion,from being ablated away by the hot, pressurized gas escaping from withinthe cell. Sleeve 1003 may be held in place using various means, such aswith one or more spacer rings 1005. Alternately, the battery packmounting substrate may be configured to maintain the position of sleeve1003 relative to the intumescent covered battery.

Intumescent layer 901 can be fabricated from any of a variety ofintumescent materials, for example, graphite-based intumescent material(e.g., expandable graphite in a polymeric binder), thermoplasticelastomers, ceramic-based intumescent material, vermiculite/mineralfiber based intumescent material, and ammonium polyphosphate basedintumescent material. Preferably the selected intumescent material has aSET temperature in the range of 100° C. to 300° C., and more preferablyin the range of 200° C. to 300° C. Preferably the intumescent materialselected for layer 901 is biologically inert, thus insuring that if theintumescent covered cell is used in an application with limited airflow,the layer's activation will be a non-toxic event.

Preferably layer 901 is formed by depositing or otherwise coating theouter surface of case 101 with the intumescent material. Alternately, asheet/strip of the intumescent material can be pre-formed, after whichthe sheet/strip is wrapped around the outside surface of the cellcasing. In this configuration, the sheet/strip of the intumescentmaterial is preferably bonded in place.

Although the embodiments described above successfully mitigate theeffects of cell wall perforations, at least some of these embodimentsmay allow gas escaping from the cell to travel between the cell wall andthe sleeve(s), exiting from either end of the cell. While the inventorshave found that this is acceptable for most applications, they envisionthat for some applications it may be preferable to limit the escapinggas to a single cell end, and preferably the same end that includes theventing element. Accordingly, for such applications, one end of thesleeve(s) is closed, using either a multi-piece or a single piecedesign. FIG. 11 illustrates a multi-piece design based on the embodimentshown in FIG. 3 that includes both a sleeve layer 1101 and a bottomlayer 1103. FIG. 12 illustrates design based on the embodiment shown inFIG. 3 in which layer 1201 covers both the sidewall and the bottomsurface of case 101. It will be appreciated that such a designconfiguration can be applied to the other embodiments described above.For example, the bottom cell surface may covered with a layer pair, thelayer pair comprised of a thermally insulating material and a high yieldstrength material; alternately, the bottom cell surface may be coveredby a high heat capacity material; alternately, a thermally insulatingmaterial may be interposed between the bottom of the electrode assemblyand the inner surface of the bottom of the cell casing; alternately, thebottom cell surface may be covered by an intumescent material layer.

Although the preferred embodiment of the invention is utilized with acell using the 18650 form-factor, it will be appreciated that theinvention can be used with other cell designs, shapes andconfigurations.

As will be understood by those familiar with the art, the presentinvention may be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof. Accordingly, thedisclosures and descriptions herein are intended to be illustrative, butnot limiting, of the scope of the invention which is set forth in thefollowing claims.

1. A battery assembly, comprising: a battery, said battery comprising: acell case having an outer surface, a first end and a second end, whereinsaid first end is closed by a cell case bottom, and wherein said secondend is comprised of a central open portion; an electrode assemblycontained within said cell case, wherein a first electrode of saidelectrode assembly is electrically connected to said cell case; and acap assembly mounted to said cell case, said cap assembly closing saidcentral open portion of said second end, wherein said cap assemblyfurther comprises a battery terminal electrically isolated from saidcell case and electrically connected to a second electrode of saidelectrode assembly; and a layer of an intumescent material surroundingsaid outer surface of said cell case.
 2. The battery assembly of claim1, wherein said battery has an 18650 form-factor.
 3. The batteryassembly of claim 1, wherein said intumescent material is selected fromthe group of intumescent materials consisting of graphite-basedintumescent materials, thermoplastic elastomers, ceramic-basedintumescent materials, vermiculite/mineral fiber based intumescentmaterials, and ammonium polyphosphate based intumescent materials. 4.The battery assembly of claim 1, wherein said intumescent material has astart expansion temperature in the range of 100° C. to 300° C.
 5. Thebattery assembly of claim 1, wherein said intumescent material has astart expansion temperature in the range of 200° C. to 300° C.
 6. Thebattery assembly of claim 1, wherein said intumescent material isbiologically inert.
 7. The battery assembly of claim 1, furthercomprising a sleeve, wherein said sleeve surrounds said intumescentmaterial, and wherein said sleeve is spaced apart from said intumescentmaterial.
 8. The battery assembly of claim 7, wherein said sleeve isfabricated from a material with a yield strength of at least 75 MPa. 9.The battery assembly of claim 7, wherein said sleeve is fabricated froma material with a yield strength of at least 150 MPa.
 10. The batteryassembly of claim 7, wherein said sleeve is fabricated from a materialwith a yield strength of at least 250 MPa.
 11. The battery assembly ofclaim 7, further comprising at least one spacer ring, wherein said atleast one spacer ring separates an interior surface of said sleeve froman exterior surface of said intumescent material prior to the thermalrunaway event.
 12. The battery assembly of claim 1, further comprising asecond layer of said intumescent material, said second layer coveringsaid cell case bottom, wherein said second layer inhibits the escape ofhot, pressurized gas from within said battery through a perforationformed in said cell case bottom during said thermal runaway event. 13.The battery assembly of claim 12, wherein said second layer of saidintumescent material is integral with said layer of said intumescentmaterial surrounding said outer surface of said cell case.
 14. A methodof preventing the formation of a perforation in an outer surface of abattery case during a thermal runaway event, the method comprising thestep of covering said outer surface of said battery case in a layer ofan intumescent material, wherein said covering step is performed priorto said thermal runaway event, and wherein said intumescent materialexpands during said thermal runaway event.
 15. The method of claim 14,further comprising the step of surrounding said layer of saidintumescent material with a sleeve, said surrounding step furthercomprising the step of separating said layer of said intumescent from aninterior surface of said sleeve by a preset distance.
 16. The method ofclaim 14, wherein said covering step further comprises the step ofcoating said outer surface of said battery case with said intumescentmaterial to form said layer.
 17. The method of claim 14, wherein saidcovering step further comprises the steps of forming a strip of saidintumescent material and wrapping said strip of said intumescentmaterial around said outer surface of said battery case.
 18. The methodof claim 17, further comprising the step of bonding said strip of saidintumescent material in place.
 19. The method of claim 14, wherein saidcovering step further comprises the step of covering an exterior bottomsurface of said battery case in said layer of said intumescent material.